This invention relates to methods of minimizing catalyst degradation during the handling of a catalyst used in a slurry phase reactor.
The use of a supported cobalt catalyst in a slurry phase reactor to produce hydrocarbon products from synthesis gas (also known as syngas) which consists mainly of CO and H2, under Fischer-Tropsch conditions, is well known in the art. Syngas may be obtained from natural gas (mainly methane) that is reformed in a steam, partial oxidation or auto-thermal reformer. The syngas is introduced into a slurry phase reactor where it is reacted over a Fisher-Tropsch catalyst to produce hydrocarbon products. The syngas H2/CO ratio is preferably controlled in a preferred range (typically between 1 and 2) by the recycle of tail gas from the slurry phase reactor or of components derived from this tail gas to a reforming unit.
The Fischer-Tropsch reaction is an exothermic reaction and requires means for heat removal and thermal control. In this regard, it may be noted that the Fischer-Tropsch reaction can be represented by the simplified equation:
CO+2H2xe2x86x92xe2x88x92CH2xe2x88x92+H2O+heat.
Heat removal and control is generally managed by internal cooling pipes in the slurry phase reactor which, in use, are submerged in the slurry phase.
Various other reactor internals which serve the purpose of ensuring catalyst suspension and introducing syngas into the reactor are also present.
Due to the nature of the reaction in a slurry phase reactor, the extreme operating conditions and high turbulence within the slurry phase reactor, there are problems with catalyst break-up and catalyst deactivation. The prior art provides various means for rejuvenation or regeneration of deactivated Fischer-Tropsch catalysts, methods of separating hydrocarbon product from catalyst particles and production of stronger catalyst particles to minimize catalyst break-up.
It is also known that catalyst is damaged during storage, transport, catalyst loading, reactor start-up and catalyst unloading from a slurry phase reactor.
It is an object of this invention to address these problems.
This invention relates to methods of minimizing catalyst degradation in catalyst handling steps such as:
catalyst loading into a slurry phase reactor;
slurry phase reactor start-up;
slurry phase reactor shut-down; and
slurry phase reactor unloading when catalyst reloading is envisaged.
The Fischer-Tropsch catalyst is protected during transport and storage by coating catalyst particles with a wax wherein the coated catalyst is in the form of discrete wax pieces each containing a plurality of catalyst particles.
The discrete wax pieces are typically in the form of cylindrical blocks.
According to a first aspect of the invention there is provided a method of loading a slurry phase reactor with a catalyst, the method including the steps of:
forming a slurry of wax and catalyst in a loading vessel;
introducing clean molten wax into the reactor; and
transferring a slurry of wax and catalyst from the loading vessel to the reactor.
Advantageously, the slurry in the loading vessel is prepared by adding a wax coated catalyst as described above to the loading vessel and heating the loading vessel to form a slurry of catalyst in molten wax.
Alternatively, the wax coated catalyst may be melted in a separate vessel prior to transfer to the loading vessel.
The loading vessel is preferably pressurized to a pressure of about 2 bar (200 kPa) above that of the pressure of the reactor to which the slurry is to be loaded, so that the slurry from the loading vessel is transferred to the reactor by the difference in pressure and there is no need to use a pump for the transfer. Typically, the loading vessel is pressurized to a pressure of 10-50 atmospheres (1000 kPa-5000 kPa), generally about 26 bar (g) (2600 kPa).
Advantageously, the loading vessel is heated to a temperature of greater than 150xc2x0 C. before the pressure within the vessel is increased to above 1 bar (g) (100 kPa) with a gas containing carbon monoxide.
Prior to introducing the slurry from the loading vessel to the reactor, the reactor preferably contains a clean molten wax and is heated to a temperature above 150xc2x0 C., typically about 160xc2x0 C., with syngas being passed through the molten wax.
The reactor may be heated by pumping a heated fluid, such as hot steam or water, through the cooling pipes thereof.
Advantageously, the syngas is recycled through the reactor in an internal recycle system.
Less than 50%, typically less than 25%, of the total quantity of catalyst to be loaded is preferably added to the reactor while syngas is recycled therethrough in the internal recycle system, while the reactor temperature is below the temperature at which the Fischer-Tropsch reaction is initiated.
The catalyst may be added in increments of, for example, about 5,0% until the desired loading of 25% to 50% is reached.
Preferably, the catalyst is added in a wax slurry, as described above, from a loading vessel with the catalyst comprising up to 70% by mass, typically from 40 to 50% by mass, of the slurry.
Advantageously, the temperature within the reactor is maintained below 200xc2x0 C. until 25% to 50% of the desired catalyst loading is reached, whereafter syngas from an external source is introduced and the temperature of the reactor is increased to about 230xc2x0 C. Typically, the syngas flow is lower than the normal operating syngas flow while reactor temperature is increased, for example, at 50% of the normal operating syngas flow.
After the temperature of the reactor is increased to 230xc2x0 C., the syngas flow may be increased, and the rest of the catalyst added. The syngas may now be introduced from an external source, with an H2:CO ratio of below 2:1.
Preferably, the flow rate of the syngas in the internal recycle system is sufficient to fully fluidize the catalyst in the slurry, prior to the introduction of syngas from the external source.
According to a second aspect of the invention there is provided a method of shutting down a slurry phase reactor, including the steps of:
stopping syngas flow from an external source to the reactor, while continuing to operate an internal recycle system;
cooling the reactor to less than 200xc2x0 C., but not less than 150xc2x0 C., while the internal recycle system continues to operate; and
transferring slurry from the reactor to an unloading vessel, while continuing to operate the internal recycle system.
Preferably, the unloading vessel is at a lower pressure than the reactor so that the slurry is transferred by the pressure difference, without the need to use a pump.
When the reactor is to be restarted, the slurry may be reloaded into the emptied reactor from the loading vessel and the reactor restarted in the manner described above.
Depending on the activity of the reloaded catalyst it may be desirable to load more catalyst than for the fresh catalyst start-up before initiating the Fischer-Tropsch reaction by increasing the reactor temperature while introducing syngas from an external source. The other features of the fresh catalyst start-up are the same for a start-up using a molten slurry of used catalyst.
According to a third aspect of the invention there is provided a method of dealing with a failure of an external recycle system of a slurry phase reactor, the method including the steps of:
stopping the supply of syngas from the external source;
maintaining an internal recycle system;
lowering the temperature of the reactor to below 200xc2x0 C.; and
unloading at least a portion of the catalyst from the reactor to an unloading vessel.
The reactor may be re-started by:
Starting syngas flow into the reactor and then increasing the temperature of the reactor to about 230xc2x0 C.; and
reloading the unloaded catalyst from the unloading vessel via a loading vessel to the reactor after attaining a stable syngas composition resulting from a stable external recycle gas composition.
According to a fourth aspect of the invention there is provided a method of dealing with failure of an internal recycle system of a slurry phase reactor, the method including the steps of:
stopping syngas flow from an external source to the reactor;
adding a quenching agent to cool the reactor while optionally simultaneously decreasing the pressure in the reactor;
maintaining the temperature in the reactor at not less than 150xc2x0 C., and not more than 200xc2x0 C., by controlling the temperature of the water that is pumped through cooling pipes in the reactor; and
optionally unloading at least a portion of the catalyst from the reactor to an unloading vessel.
The reactor may be re-started by:
starting the internal recycle system;
if necessary, pressurizing the reactor to the normal operating pressure of between 8 and 48 atmospheres (800-4800 kPa), typically about 24 bar (2400 kPa);
heating the reactor by pumping heated fluid through the cooling pipes of the reactor;
introducing syngas from an external source when the catalyst in the reactor is fluidized by the internal recycle system and then increasing the temperature of the reactor to about 230xc2x0 C.; and
if necessary, reloading the unloaded catalyst from the unloading vessel via a loading vessel to the reactor after attaining a stable syngas composition from the external syngas source.